Porous sorbents are currently employed in a variety of fields, including gas storage, separations, filtration, catalysis, and others. The ability to introduce active sites onto sorbents that are capable of reaction with both acidic and basic gases continues to be an area of interest. Such sorbents are incorporated into respirators used by emergency and/or military personnel to remove toxic chemicals. For example, the National Institute for Occupational Safety and Health (NIOSH) requires chemical, biological, radiological, and nuclear (CBRN) filters to provide protection against a wide variety of gases, including ammonia (NH3), chlorine gas (Cl2), and cyanogen chloride (CNCl). Current military and commercial individual (small) and collective (large—building/vehicle) filters employ activated, impregnated carbon. Although carbon is typically an excellent material for removing highly toxic chemicals such as nerve, blister, blood, and choking agents, there are shortcomings with gases beyond those toxic chemicals.
To this end, metal-organic frameworks (MOFs) have been intensely studied for a variety of potential applications, including gas storage, catalysis, separations, and toxic gas removal. MOFs are crystalline materials built using metal containing secondary building unit (SBU) clusters and organic linkers, which can be functionalized. The ability to change the SBU and organic linker allows for a large potential number of combinations and therefore performance and properties. Many MOFs are inherently microporous; however, the linkers can be extended or templates can be used to make mesoporous MOFs. Micropores refer to pore sizes less than 2 nanometers, while mesopores refer to pore sizes between 2 and 50 nanometers. The ability to incorporate multiple functionalities into the backbone of MOFs is a major advantage over traditional activated, impregnated carbons. However, for use in filtration and other applications where diffusion kinetics are important, a wide range of pore sizes are sometimes required. Micropores are necessary for high-energy adsorption sites, while mesopores and macropores are required for transport from the bulk (air) phase to the active sites.
One potential shortcoming of MOFs is that they are almost exclusively microporous, and in dynamic separations, small pore apertures may lead to reduce mass transfer. Attempts to make isostructural MOFs with larger pores has proven successful with extension of the organic linker. However, the modification may lack sufficient stability and does not possess a hierarchical pore structure. Other MOFs have been synthesized with mesopores, many of which require the use of a templating agent. Only a few, if any, MOFs possessing a hierarchical combination of micropores and mesopores are known. Furthermore, there is a long felt need for a post-synthetic method to develop a hierarchical pore structure within MOFs.